457
FXUS66 KMTR 030457
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
957 PM PDT Sat Aug 2 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 1245 PM PDT Sat Aug 2 2025

 - Elevated breezes across channels, gaps, passes, and across the
   marine environment through Monday.

 - A warming trend; Moderate HeatRisk Wednesday and Thursday
   across interior reaches of the North and East Bay.

&&

.UPDATE...
Issued at 842 PM PDT Sat Aug 2 2025

Satellite imagery shows dry air filtering in as the stratus bank
clears from north to south. Low clouds may still develop in the
North Bay, but most of the night will likely remain clear north of
the Golden Gate as this drying trend continues. The 00Z balloon
from Oakland found a significant dew point depression throughout
the marine layer, although the latest trend has stratus filling
in across the Peninsula and East Bay, so that dry air hasn`t held.
Further South along the Central Coast, the marine layer is much
more robust and the typical nocturnal stratus will certainly
persist through mid morning.

&&

.SHORT TERM...
Issued at 1245 PM PDT Sat Aug 2 2025
(This evening through Sunday Night)

Increased breezes as well as patchy drizzle/fog are forecast
tonight into Sunday. While the marine layer may compress a little
through the day today, the westerly mid-level wind field does seem
to increase. This should encourage more in the way of onshore
flow tonight. There is a response in the near surface layer winds,
which may promote more in the way of drizzle as opposed to fog,
especially where orographic ascent is optimized.

With the increase in the wind field, the primary story over the next
24 to 36 hours will be the enhancement of the surface flow across
the wind corridors (channels, gaps, and passes), as well as across
the immediate shore line downwind of Point Arena and Point Sur. As
noted previously, a majority of the model guidance has been
underwhelming for some of our windier locations. The NCEP short term
hi-res model consensus appears to be a good starting point for these
wind/wind gust fields. Our latest MTRWRF as well as experimental
National Blend of Models output also appear a little more inline
with these tools compared to the operational NBM and GFS. Examining
the Extreme Forecast Index (which is a measure of how "different"
the ECMWF ensembles are with respect to their model climatology)
indicates "stronger than normal" wind forecast across the coastal
waters and portions of the East Bay interior. This yields increased
confidence in coastal jets and enhanced flow across the complex
terrain transpiring in the short term period.

Overnight into Sunday morning, coastal jets will ramp up near/south
of Point Arena and Point Sur where winds have the potential to
exceed 40 knots (45 mph) in spots. Adjacent coastal regions may
experience wind gusts close to 35 mph. Across the East Bay interior
(near Altamont Pass) and downwind of the Santa Cruz Mountains (San
Bruno Gap) winds will accelerate around terrain features into the
afternoon. With some stability, there could be some "mountain"
wave like activity as flow is ducted beneath the stable layer
across the lee side of both terrain features. The elevated winds
may translate to hazardous boating conditions across the coastal
waters. For more on that, see the marine section below. While
Energy Release Components or ERC values don`t support a high
probability for fire spread, anything that gets going in one of
the wind corridors could exhibit some resistance to control. Wind
gusts abate briefly in some spots during the afternoon as mixing
redistributes the momentum through the boundary layer. The
exception may be for areas downwind of the San Bruno Gap. The
onshore flow will likely keep Sunday MaxTs a few degrees lower
than today (Saturday). Highs along the coast are anticipated to
fall between 55 and 70 degrees, with interior regions averaging
between 70 and 85 degrees. Far southern portions of Monterey
County appear to be on track for high temperatures above 95F.

&&

.LONG TERM...
Issued at 1245 PM PDT Sat Aug 2 2025
(Monday through next Friday)

Enhanced winds near/south of Point Arena and Point Sur (coastal
jets) and across the East Bay interior are still anticipated through
at least noon on Monday. The onshore flow will keep temperatures
in check and very much near what was noted for Sunday.

The extended forecast period will see a slight change from our
relatively cooler summer that we`ve experienced thus far. While it
doesn`t seem probable that we`ll see an extreme heat event mid-
week, temperatures will rebound to around normal, with some sites
near +10F above climatological normals. This results in pockets
of Moderate HeatRisk, especially across the interior portions of
the Bay Area and Central Coast. The deterministic NBM appears to
be a realistic starting point for the forecast. However, there are
some cooler and hotter solutions that reside in some of the
forecasts. The cooler solutions, as we`ve seen this past summer,
advertise more persistent troughing which will keep the Four
Corners high anchored over the Pacific Southwest. Again, that has
been the theme for this year and these larger scale patterns often
require some dramatic feature to shift things. Not seeing
anything in the guidance that would suggest that yet. It should be
noted, that our cooler than normal summer may result in a slight
cool bias in the deterministic NBM. The warmer solutions weaken
the aforementioned troughing such that low/mid level flow
diminishes, effectively limiting the amount of inland intrusion of
the marine layer. Should the troughing be less diffuse and/or the
Four Corners high move farther to the west, then temperatures
(mainly across the interior) could approach or exceed 100F. With
the Central Valley likely to heat up, this should drive diurnal
wind patterns that will keep the coastal regions very temperate
thanks to the marine stratus. If the warmer conditions do verify,
then we`ll examine an increased potential for grass fires as
fuels become cured and available. Fortunately, it doesn`t appear
that we have any significant wind potential (outside of the gaps,
passes, channels) with this synoptic scale pattern.

One last thing to mention and that`s eastern Pacific tropical
activity. Climatologically, these have been an important player
for some of our fires induced by lightning events in August. At
this time, the train of tropical cyclones appears that it`ll
remain far south enough such that there will be little to no
influence. We`ll need to watch the position of the coastal trough,
Four Corners high and subsequent mid/upper level moisture feeds.

&&

.AVIATION...
(06Z TAFS)
Issued at 934 PM PDT Sat Aug 2 2025

Drier air out ahead of an approaching trough should keep the North
Bay terminals VFR through the forecast period, however there is a
chance for KAPC to develop MVFR/IFR ceilings during the overnight
hours. Gusty onshore winds, especially across the Bay Area
terminals will set in late Sunday morning and persist through the
remainder of the TAF period.

Vicinity of SFO...Low stratus continues to linger in vicinity of
the Bay Area terminals despite the drier air to the north. HiRes
guidance indicates VFR conditions will persist through much of the
overnight with MVFR/IFR ceilings close by. Gusty onshore winds
will develop late Sunday morning and persist through the remainder
of the forecast period.

SFO Bridge Approach...Low MVFR/IFR stratus ceilings may be present
even if KSFO is VFR.

Monterey Bay Terminals...IFR/LIFR ceilings are expected to persist
overnight through Sunday morning with VFR developing in the
afternoon, later at KMRY. Gusty onshore winds will set in Sunday
afternoon and ease after sunset.

&&

.MARINE...
(Tonight through next Friday)
Issued at 842 PM PDT Sat Aug 2 2025

Fresh to strong northwesterly breezes with near-gale force gusts
will continue through the weekend into the beginning of next week.
The coastal jet regions of Point Reyes and Point Sur can expect
at isolated gale force gusts. Expect hazardous conditions for
small craft in the bays during the afternoon and evening hours,
especially Sunday and Monday. Seas will remain moderate to rough
through the middle of next week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM Sunday to 3 AM PDT Monday for SF
     Bay N of Bay Bridge.

     Small Craft Advisory from 9 AM Sunday to 3 AM PDT Monday for Mry
     Bay-Pigeon Pt to Pt Pinos 0-10 nm.

     Small Craft Advisory until 3 PM PDT Sunday for Pt Arena to Pt
     Reyes 0-10 nm.

     Gale Warning from 3 PM Sunday to 3 AM PDT Monday for Pt Arena to
     Pt Reyes 0-10 nm.

     Small Craft Advisory until 3 AM PDT Monday for Pigeon Pt to Pt
     Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM-Pt Reyes to
     Pigeon Pt 0-10 nm.

     Small Craft Advisory until 3 AM PDT Sunday for Pigeon Pt to Pt
     Pinos 0-10 nm.

     Small Craft Advisory until 9 AM PDT Sunday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

     Gale Warning from 9 AM Sunday to 3 AM PDT Monday for Pt Pinos to
     Pt Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...Bain
LONG TERM....Bain
AVIATION...JM
MARINE...JM

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